1. Field of the Invention
The present invention relates to a novel sulfonium salt compound, a method for producing the same, and a photoacid generator containing the sulfonium salt.
2. Background Art
Conventionally, sulfonium salt compounds have been used for various applications, such as for a photoacid generator to be used for chemically amplified resist materials. Such a chemically amplified resist material contains a resin, a photoacid generator, and a solvent. The chemically amplified resist material after being applied is irradiated with radiation such as an electron beam and X-ray within a desired region of the above-applied chemically amplified resist pattern. Thus, in response to the irradiated radiation, the photoacid generator generates an acid and the generated acid changes the solubility of the resin, which allows a resist pattern for creating an integrated circuit to be formed.
Further, integrated circuits are microfabricated in recent years. Therefore, there has been a demand for a photoacid generator having a high resolution and capable of forming a pattern with sharp edges (excellent pattern profile) in order to form a fine-scale resist pattern. As one of factors that affect the resolution and pattern profile, the acid diffusion length of the photoacid generator can be mentioned. When this acid diffusion length is large, the resolution and the precision in the pattern profile are reduced, which is commonly known.
Therefore, there is proposed a technique for suppressing the acid diffusion, for example, by employing an anion with a comparatively large volume as the anion of the photoacid generator or by introducing a polar group into the anion (see JSR TECHNICAL REVIEW No. 118, p. 8-13 (2011)). However, these methods affect the acidity or properties of the anion, and therefore there may be cases of failing to achieve the desired acidity or properties of the anion.
Then, there is proposed a photoacid generator configured to contain two kinds of sulfonium salt compounds, one of which has a comparatively strong acid anion (for example, fluorine-substituted sulfonylimide), and the other of which has a comparatively weak acid anion (for example, sulfonic acid or carboxylic acid having no fluorine substitution) (see JP 2008-7410 A). In such photoacid generator, a strong acid generated from the sulfonium salt compound having a comparatively strong acid anion by irradiation with radiation collides with the sulfonium salt compound having a comparatively weak acid anion, which has not been reacted, and this collision causes a salt exchange. As a result, the weak acid is released, whereas the released strong acid is incorporated into the sulfonium salt compound as a part thereof. Thus, the strong acid generated by irradiation with radiation is exchanged with the weak acid having a lower catalytic performance, which causes apparent inactivation of the acid, resulting in suppression of the acid diffusion.
Further, there is proposed a sulfonium salt compound having 10-camphor sulfonate anion as the sulfonium salt compound having a comparatively weak acid anion (JP 2010-215608 A). Such 10-camphor sulfonate anion is less likely to diffuse because of its high bulk structure. Accordingly, this sulfonium salt compound can suppress the acid diffusion. However, such a sulfonium salt compound generally does not have a sufficient solubility in propylene glycol 1-monomethyl ether 2-acetate (PGMEA) that is widely used for chemically amplified resist materials.
Meanwhile, i-line radiation at a wavelength of 365 nm is widely used for formation of resist patterns with large thickness using a photoacid generator. One of the reasons for that is availability of light sources such as a high-pressure mercury lamp and a metal halide lamp that allow good emission intensity of i-line light despite its low cost. Recent widespread adoption of LED lamps with an emission wavelength in the i-line region (360 to 390 nm) also can be mentioned as another reason. Molecular extinction coefficient (e) at 365 nm (i-line) is one of indicators for responsiveness to the i-line light.
From these reasons, photoacid generators are required to exhibit a sufficiently high responsiveness to the i-line light, as well.
However, the responsiveness is not necessarily improved simply by increasing the molecular extinction coefficient (ε) at the i-line. For example, a sulfonium salt compound into which thioxanthone skeleton is introduced is proposed (JP 8 (1996)-165290 A); however, the sulfonium salt compound absorbs light mostly on the side of the surface on which the resist material is applied because of its excessively high molecular extinction coefficient (ε) at 365 nm (i-line), as a result of which the light does not penetrate into a deep portion, and thus the acid generation efficiency rather tends to decrease.
Therefore, a sulfonium salt compound having a naphthalene ring in a cationic group is proposed as being useful, for example, as a photoacid generator for chemically amplified resists (see JP 2004-334060 A, JP 2006-276755 A, and JP 2010-256168 A).